1. Field of the Invention
The present invention relates to an antenna structure provided for a wireless communication device, such as a cellular phone, and a wireless communication device having the antenna structure.
2. Description of the Related Art
FIG. 9 is a schematic perspective view of an example of an antenna structure (for example, see Japanese Unexamined Patent Application Publication No. 2006-203446). The antenna structure 40 has an antenna element 41. The antenna element 41 is defined by a dielectric base 42 and a feeding radiation electrode 43. The feeding radiation electrode 43 is provided on the dielectric base 42 and operates as an antenna. The feeding radiation electrode 43 has a slit S. Due to the slit S, the feeding radiation electrode 43 has a long electrical length extending from a feeding portion Q, which defines one end of a current path of the feeding radiation electrode 43, to an open end K, which defines the other end, as compared to the case in which no slit S is provided. Thus, by elongating the electrical length, the size of the feeding radiation electrode 43 is reduced, while the feeding radiation electrode 43 may have an electrical length with which the feeding radiation electrode 43 resonates at a predetermined wireless communication frequency band.
The antenna element 41 is, for example, mounted in a non-ground region Zp of a circuit board 44 of a wireless communication device. The circuit board 44 has a ground region Zg in which a ground electrode 45 is provided and the non-ground region Zp in which no ground electrode 45 is provided. The antenna element 41 is mounted on the non-ground region Zp. When the antenna element 41 is mounted at a predetermined position in the non-ground region Zp, the feeding portion Q of the feeding radiation electrode 43 is electrically connected to a wireless communication circuit 47 through a feeding line 46 provided on the circuit board 44.
In the antenna structure 40, for example, when a wireless transmission signal is supplied from the wireless communication circuit 47 to the feeding radiation electrode 43, the feeding radiation electrode 43 resonates and then the wireless transmission signal is wirelessly transmitted. In addition, when a signal arrives and the feeding radiation electrode 43 resonates to receive the signal, the received signal is transferred from the feeding radiation electrode 43 to the wireless communication circuit 47.
Incidentally, in recent years, miniaturization has been required, particularly, for a wireless communication device, such as a portable mobile terminal with wireless communication function (for example, cellular phone). Because of this requirement, miniaturization is also required for the antenna structure. To miniaturize the antenna element 41, the feeding radiation electrode 43 also needs to be miniaturized. However, when the feeding radiation electrode 43 is miniaturized, the electrical length becomes insufficient and, therefore, the resonant frequency of the feeding radiation electrode 43 cannot be decreased to a desired frequency. As a result, the feeding radiation electrode 43 is not able to wirelessly communicate in a predetermined wireless communication frequency band. Thus, to miniaturize the feeding radiation electrode 43, it is necessary to take some measures to elongate the electrical length.
As an example of such measures, as shown in FIG. 9, the feeding radiation electrode 43 has a meandering shape, or other suitable shape, to elongate the physical length from the feeding portion Q to the open end K, thus elongating the electrical length. When these measures are used, the shape of the feeding radiation electrode 43 is complex and, in addition, the path width of the feeding radiation electrode 43 is relatively narrow. A narrow path width problematically causes an increase in conduction loss and, as a result, the efficiency of the antenna deteriorates. In addition, with a complex shape, a problem arises in that it is difficult to adjust the resonant frequency of the feeding radiation electrode 43.
In addition, with the configuration of the antenna structure 40 shown in FIG. 9, in addition to the problems related to miniaturization, the following problems also exist. That is, the antenna element 41 is mounted on the circuit board 44, such that the antenna element 41 is arranged adjacent to the ground electrode 45 that is required for the circuit board 44. Then, the electric field of the feeding radiation electrode 43 is attracted toward the ground electrode 45 to increase the Q value. For this reason, there is a problem in that it is difficult to provide a wide frequency band for wireless communication.
In addition, for example, a hand that is holding or operating a wireless communication device (for example, cellular phone) may be located near the feeding radiation electrode 43. The hand functions as a ground and, therefore, a stray capacitance is formed between the feeding radiation electrode 43 and the hand. Due to the stray capacitance, there is a problem in that the antenna characteristic fluctuates or degrades to reduce the reliability to wireless communication.